The present invention relates to an improved high pressure electric discharge lamp.
DT-OS No. 24 22 411 discloses the filling for a high pressure mercury vapor discharge lamp containing mercury and at least one of the halogens, iodine, bromine, chlorine; a halide of at least one of the metals from the group of alkali and alkaline earth metals, and, if desired, of cadmium, gallium, indium, thallium, tin, scandium, yttrium, and rare earth metals; and also a volatile halide. Aluminum halide, or a aluminum halide together with a halide of trivalent iron, bivalent tin and/or trivalent indium, are disclosed as the volatile halide. The numerous examples disclose that usually aluminum chloride is added, less frequently the bromide.
Because of the presence of the nonvolatile alkali metal halides and alkaline earth metal halides, the lamp under normal conditions does not contain sufficient halide in the vapor state, which adversely affects the luminous efficacy and color rendering of the lamp. The volatile aluminum chloride which together with the nonvolatile alkali metal halides and alkaline earth metal halides forms a gaseous compound, is used with the intention to thereby increase effective partial pressure of the nonvolatile compounds without increasing the thermal load on the arc tube wall. Said DT-OS No. 24 22 411 also discloses lamps having a filling of sodium iodide and the tin iodide, with or without aluminum chloride. From the foregoing, it follows that the addition of tin iodide to the sodium iodide without aluminum chloride will not result in a lamp of satisfactory luminous efficacy. The electrodes of the lamps are spaced a long distance of 40 mm which is almost to three times the inner diameter of the tube, so that the lamp is not electrode-stabilized.
The use of a volatile complex halide compound of tin and sodium has become known from DT-OS No. 26 05 290, wherein as disclosed in the examples, the chlorine is always the halogen present in the filling. Iodine, on the other hand, is never used as the only halogen. The color temperature of said lamps does not exceed 3900-4600 K. In the examples, the electrodes are spaced 20 mm apart. It can be concluded from the long distance separating the electrodes, that the lamps are wall-stabilized and not electrode-stabilized.
Another lamp has already been proposed which contains for excitation of molecular emission the preferred mercury and the rare earth metals and, in addition, alkali metal halide and alkaline earth metal halide, thallium halide and tin halide.
The present invention seeks to provide a lamp which, while exhibiting high luminous efficacy (&gt;70 lm/W) and low color temperature (below 3800 K), has a good color rendering (general color rendering index according to CIE of R.sub.a &gt;70), the least possible power input, independence of position, a long service life, is suitable for use for interior lighting.